High-resolution laser-induced fluorescence spectroscopy of ²⁸Si¹⁶O⁺ and ²⁹Si¹⁶O⁺ in a cryogenic buffer-gas cell

Optical cycling has been successfully applied in many fields of atomic research, including laser cooling, atom trapping and quantum information, and therefore inspires the extension to molecular systems, such as SrF, CaF, YbF, YO, SrOH, CaOH, YbOH and CaOCH₃. However, few experiments have been done on molecular ions while several candidates have been proposed, including SiO⁺, BH⁺, AlH⁺, AlCl⁺, BO⁺, PN⁺ and YF⁺. ²⁸Si¹⁶O⁺, one of the most promising candidates, is found to have a highly vibrationless decay for the B²Σ⁺ → X²Σ⁺ transition and has been rotationally cooled by the Odom group. In this work, we investigate the suitability of hosting a hyperfine qubit in its isotopologue, ²⁹Si¹⁶O⁺. We have carried out the high-resolution laser-induced fluorescence spectroscopy of ²⁹Si¹⁶O⁺ and ²⁸Si¹⁶O⁺ in a cryogenic buffer-gas cell at around 90 K and obtained the corresponding rotational and hyperfine constants. The quantum logic operations using the hyperfine structures of ²⁹Si¹⁶O⁺ are discussed and an electric field gradient gate (EGG) is proposed. Simulations indicate that the fidelity of the two-qubit gate is >0.99 with the gate time on the order of a few milliseconds. Our results show that ²⁹Si¹⁶O⁺ has great potential in quantum information science.